This invention relates to filter materials for filtering fluids and methods for the production thereof, with particular, but by no means exclusive, reference to filters for air filtration.
Air filter materials having permanent electrostatic charge are known. Such materials filter particles very efficiently due to electrostatic attractive forces between the material and the particles to be filtered. Applications include air filtration in dust helmets, respirators, suction cleaners and air conditioning systems.
Filter materials desirably possess a number of advantageous properties. Such materials should filter particles efficiently whilst providing low resistance to the flow of fluid through the filter material. In the case of air filtration, this means a low air pressure drop across the filter material. Furthermore, it is important that the filtering performance of the filtering material is maintained, or, at least, that deterioration in performance is minimised, for as long a period of time as possible. In this way, the working lifetime of the filter is extended. Furtherstill, it is important that once the working lifetime of the filter is over, the filter can be disposed of safely. Recently, there has been a demand for filter materials which are resistant to attack by oily substances.
European Patent EP 0 246 811 describes air filter material which exhibit good filtration efficiency and low resistance to air flow due to the open structure of charged fibres employed. The material comprises a blend of polyolefine fibres and halogen substituted polyolefine fibres. U.S. Pat. No. 5,470,485 describes air filter material comprising a blend of bi-component polypropylene/polyethylene fibres of the core/jacket or side-by-side type and halogen free polyacrylonitrile fibres. The use of the bi-component fibres is an essential element of U.S. Pat. No. 5,470,485 so that stiff, mouldable non-woven mats of filter material can be produced. It should be noted that in each case the filter material is overall electrically neutral, fibres in the materials of different type carrying charges of opposed polarities.
The present invention provides high quality filter material which exhibits some or all of the advantageous properties described above.
According to a first aspect of the invention there is provided an electrostatically charged filter material comprising a blend of polyester fibres and electrostatically chargeable fibres of at least one other kind.
Such filter materials can exhibit excellent filtering efficiency, and shelf-lifetime, even if exposed to oil based liquid aerosols. Furthermore, polyester is readily available, and does not produce dioxins on incineration, in contrast in the halogen containing blends disclosed in EP 0 246 811.
Preferably, the filter material consists of a blend of polyester fibres and polyolefin fibres. In contrast to the filter material of EP 0 246 811, such material may be safely disposed of by incineration, since dioxin compounds are not produced. Preferably, the polyolefin fibres comprise polypropylene. The denier of the polypropylene fibres may exceed the denier of the polyester fibres. The denier of the polypropylene fibres may exceed the denier of the polyester fibres by a factor of at least 1.5.
The ratio by weight of the polyester fibres to the fibres of the other kind or kinds may be in the range 80:20 to 20:80; preferably 70:30 to 30:70; most preferably 60:40 to 40:60.
According to a second aspect of the invention there is provided a method for producing an electrostatically charged filter material comprising the step of blending together polyester fibres and electrostatically chargeable fibres of at least one other kind, and making a filter material from said blend.
The performance of the filter material is considerably affected by the present of lubricants and anti-static agents (collectively known as a xe2x80x9cspin finishxe2x80x9d) on the surface of the fibres. In practise, polymer fibres produced commercially have spin finishes, and thus it is necessary to substantially remove lubricants and anti-static agents from the fibres before, during or after the blending of said fibres.
The method may comprise the steps of carding and/or air-laying, and cross-lapping the blend of fibres so as to form a non-woven fabric. The non-woven fabric may be suitable. Mechanically/thermally bonded, by methods such as calendering or needle-punching.
The polyester fibres may be blended with polyolefin fibres, which may be polypropylene fibres.